RU2105008C1 - Method of preparing rhodium (iii) and iridium (iii) beta- diketonates - Google Patents

Abstract

FIELD: non-ferrous metallurgy, more particularly preparation of volatile rhodium and iridium complexes. SUBSTANCE: claimed method comprises using complex fluorides of said metals in oxidation state (III). Said complexes are produced by reaction of fusion of hexanitrites thereof with mixture of potassium hydrofluoride and acid at certain ratio as original compounds for reaction with beta-retones under appropriate conditions. Amounts of original compounds, cheap reagents, standard equipment and few operations make it possible to simplify method of preparing rhodium and iridium beta-diketonates and retain high yield thereof. EFFECT: more efficient preparation method. 3 cl

Description

The invention relates to the field of coordination chemistry, where organic molecules act as ligands, and noble metals, in particular rhodium and iridium, play the role of a central atom. These compounds have both theoretical significance - that is, model salts of a general form:
(R ¹ -CO-CH-CO-R ¹¹ ) ₃ M
where R ¹ = R ¹¹ - CH ₃ (acetylacetonate);
R ¹ -CF ₃ , R ¹¹ -CH ₃ (trifluoroacetylacetonate);
R ¹ = R ¹¹ - CF ₃ (hexafluoroacetylacetonate).

in which the metal has a chelated octahedral environment with oxygen atoms, and practical applications. The fields of application of these complexes are based on such exceptional properties as the strict stoichiometric metal-ligand ratio and the ability to sublimate at relatively low temperatures. The latter circumstance is used to obtain films of these refractory metals (mp 2410 ^o C and 1966 ^o C iridium and rhodium, respectively) by thermolysis in a low temperature range of 300-400 ^o C. In the absence of the traditionally used electrochemical methods for obtaining these metals on various metal substrates the method of gas-phase deposition of rhodium and iridium may become the most promising. The main factor hindering the widespread use of these compounds in practice is the lack of reliable methods for their synthesis, allowing to obtain these coordination compounds in high yield.

 The chemical essence of the chelation process is to obtain uncharged complexes with weakly coordinated oxygen ligands of acetylacetone. Taking into account the competition of other ligands for the internal sphere of rhodium or iridium, as well as the ability of acetylacetone to bind due to the carbon atom, which leads to the formation of charged complexes, the low yield of the target products becomes clear.

 Available in the scientific and patent literature, methods for isolating these tris-chelate complexes are not without drawbacks. The main one, in our opinion, is the use of reagents traditionally known from the chemistry of these compounds. It is this fact that leads to the target synthesis by a multi-stage process, resulting in a low yield of the product. According to the authors of [1, 2], who used freshly precipitated iridium hydroxide released during the reduction of its hexachlorocomplexes, it does not exceed the value of 15%. The explanation for this is the high degree of polymerization of hydroxides, its low stability to oxidation by atmospheric oxygen, and attempts to conduct the reaction in the presence of a reducing agent do not lead to an increase in yield. In this regard, as follows from [3], it becomes necessary to carry out rather difficult stages and maintain some special experimental conditions. First of all, these are lengthy heating procedures (40 h) of iridic acid or iridium trichloride, the use of a large excess of acetylacetone (10-fold over stoichiometry), the maintenance of an inert gas atmosphere, and the isolation of the desired fraction by extraction with dichloromethane. All this makes these methods unproductive, time-consuming, economically inefficient and, ultimately, low-tech.

 The closest known method according to the technical essence of the solution and the final result is the method [4], the authors of which came closest to understanding the intimate mechanism of chelation and factors influencing it. Using fluorine-containing complexes characterized by a low metal-fluorine binding energy in solutions, or their aquated forms, they excluded the competitive effect of the ligands of the initial salt on the process of obtaining chelates. As a result of this, the chemical yield of the necessary salts rose sharply.

 However, the way to achieve such values for the use of precious metals still has a number of significant drawbacks: the first one is to use elemental fluorine - an extremely aggressive gas that requires special equipment to work with it, increased safety requirements, and trained personnel; the second follows from the high oxidizing ability of fluorine, which implements the hexafluorocomplexes of rhodium and iridium in higher oxidation states, respectively (IV and V). This circumstance requires additional operations to obtain fluorocomplexes of these metals in the required degree of oxidation III. Thus, it becomes necessary to search for other fluorinating agents that would not affect the initial oxidation state of these metals in their complex salts. In this regard, the objective of the invention was to increase the yield of target products and simplify all stages of the process through the use of hexafluorocomplexes of rhodium and iridium, immediately having an oxidation state of (111). This problem is solved in that the used fluorocomplexes of iridium and rhodium are obtained by the reaction of fusion of their hexanitrite salts with a mixture of potassium bifluoride with hydrofluoric acid in certain ratios. Then, the resulting salts are dissolved in hydrofluoric acid and hydrolyzed by heating. After that, neutralization is carried out and a solution of the corresponding beta-diketone is introduced, adjusting the pH of the solution for the maximum possible isolation of the final product from the solution.

 The main distinguishing features: the use of hexafluorocomplexes of iridium and rhodium in the required degree of oxidation, the method of their preparation, as well as the modes and sequence of all stages of the process. It is to obtain complex fluorides in the oxidation state (111) that there is a need to search for fluorinating agents that would not affect the initial oxidative state of metals.

При этом отмечается, что простое выпаривание нитритов родия и иридия с плавиковой кислотой не приводит к полному замещению нитритных групп. Рекомендуемое в литература соотношение комплексного нитрита к бифториду калия при проведении сплавления в 5 раз превышает стехиометрическое, что связано с нерациональным использованием выделяющегося в плаве фтористого водорода.In preparative and technological practice, anhydrous hydrogen fluoride is used to solve such problems. It does not have oxidizing properties, but working with it also presents known experimental difficulties. A solution to these questions can be found by using alkali metal hydrofluorides as fluorinating agents. In their reactivity, they approach anhydrous hydrogen fluoride, but at the same time they have low melting and boiling points, which does not require the use of high temperatures. Bifluorides (hydrofluorides) of alkali metals are relatively cheap products produced by the industry in large quantities. The melting point of potassium bifluoride, the most convenient fluorinating reagent, is 238.7 ^o C. Its melting has a congruent character and is largely dissociated in the melt. The density and viscosity of bifluoride melts decreases linearly with increasing temperature. The thermal stability of potassium bifluoride lies in the range of the onset of decomposition of 400 ^o C and the final removal of HF at 500 ^o C. Thus, fluorination of potassium bifluoride complex compounds of rhodium and iridium can lead to the desired fluorocomplexes in oxidation state 111. The most suitable complex salts for this purpose noble metals, as shown by analysis of the literature [5], will be their hexanitrite. When the latter is fused with potassium bifluoride as a result of the exchange reaction of the nitrite complex, the corresponding fluorocomplex is formed by the reaction:

It is noted that the simple evaporation of nitrite of rhodium and iridium with hydrofluoric acid does not lead to the complete replacement of nitrite groups. The ratio of complex nitrite to potassium bifluoride recommended in the literature during fusion is 5 times higher than stoichiometric, which is associated with the irrational use of hydrogen fluoride released in the melt.

 The essence of the proposed method is to reduce the amount of hydrofluoride to 1.5 over stoichiometry, while introducing certain volumes of 40% hydrofluoric acid. The optimal ratio of bifluoride to hydrofluoric acid lies in the ranges T: W (solid to liquid) as 1: 0.5-1.0. The lower limit of the amount of hydrofluoric acid introduced guarantees the full depth of the fluorination process at given temperatures and modes of its rise. The upper range is determined by the resistance of the resulting complex fluorides to hydrolysis, which will decrease as the water introduced with the acid increases.

 Thus, a new set of essential features arises: the fluorination of the initial complex soda in the potassium bifluoride-hydrofluoric acid system, followed by hydrolytic replacement of fluorine ligands with water molecules in an acidic medium. This new set of features leads to a new technical solution - a more complete fluorination process in less time with significantly less potassium bifluoride, which dilutes the final product and requires the stages of its removal.

 As a result, using fluoride complexes of the corresponding metals to the desired degree of oxidation and conducting their preliminary hydrolysis, it is possible to transfer these metals with a high yield into almost the only complex form required. In addition, in the process of performing these operations, the need to work with aggressive fluorine is eliminated, in addition to obtain the desired fluorine-containing compound. The fusion process is carried out in standard equipment, and the required platinum crucibles are replaced by available glassy carbon. All this ultimately greatly simplifies the method of obtaining beta-diketonates of these metals and allows them to be obtained in high yield.

Example 1. A portion of potassium hexanitro-rhodiate weighing 50 g (0.1 mol) obtained in a known manner [6] is mixed with potassium bifluoride weighing 71 g and thoroughly mixed in a porcelain mortar. The resulting mixture is placed in a glass with a volume of 600 ml of glass carbon brand SU-1000, to which 40% hydrofluoric acid is then added in a volume of 40 ml. This pulp is heated in the muffle to a temperature of 300 ^o C and maintained for 0.5 hour until the emission of brown nitrogen oxides ceases, after which the sintering temperature rises to 500 ^o C and after 10-20 minutes the heating is stopped. The melt cooled in the muffle is leached with water, which dissolves the excess KF, and the water-insoluble hexafluororodiate is filtered on a fluoroplastic funnel through a paper filter. The beige or pink precipitate is dissolved in 100-120 ml of 40% hydrofluoric acid and the resulting solution is heated to boiling and boiled for 2-3 hours until it turns orange, adding distilled water as it boils. The resulting solution, where its dominant form of rhodium (III) is its aqua-ion, is neutralized with 1M KOH to pH 3-5, and then 90 ml of acetylacetone is introduced, which is a 3-fold excess of stoichiometry. The mixture is stirred vigorously and after 5-10 minutes a yellow-orange-colored rhodium acetylacetonate precipitate begins to precipitate. The solution is cooled and the solid phase is filtered off, dried and weighed. The mother liquor is made alkaline to pH 5-7 and acetylacetonate is extracted with several portions of benzene until the color of the solution ceases. After distillation of the solvent, the precipitates are combined and sublimated at a temperature of 150 ^o C and a vacuum of 10 ^-3 Torr. The yield of the final product, which is calculated from the mass value of the sublimated salt and additional analysis of rhodium in the aqueous phase, reaches 92%.

Example 2. A sample of white potassium hexanitroiridate synthesized according to the method [4] in 60.9 g (0.1 mol) is thoroughly ground in a porcelain mortar with potassium bifluoride taken in an amount of 71 g. The mixture is transferred into a 600 ml glassy carbon crucible and after adding 50 ml of hydrofluoric acid, 40% concentration is heated in a muffle furnace at a temperature of 400 ^o C for one hour. After cooling, the alum-red cake is leached with hot hydrofluoric acid and the solution is filtered. The filtrate is heated to a boil and the solution turns purple, which turns into an olive green after adding a few drops of 60% hydrazine hydrate. The resulting solution of potassium hexafluoroiridate is boiled for 2 hours and its hydrolyzed fluoro-aqua forms are capable of reaction with acetylacetone. To do this, after adjusting the pH of the solution to a value of 1-3 using KOH, an alcoholic solution of acetylacetone in an amount of 90 ml is introduced, which is a three-fold excess relative to the required stoichiometry 1: 3. The reaction mixture is heated with stirring in a fluoroplastic flask equipped with a reflux condenser for half an hour. After that, the pH of the solution is adjusted, which drops due to the formation of Tris-chelate with the KOH solution to a value of 3-5, the precipitate of the product is filtered off and dried. The filtrate is reheated and heated, and the remaining portion of beta-diketonate is extracted with benzene. The percentage distribution of metal on average is: solid phase 70-80, benzene extracts 16-20, aqueous phase 6-12. The yield of iridium tris-acetylacetonate after purification by sublimation or recrystallization from benzene lies in the range of 88-94%.

The synthesis of rhodium and iridium beta-diketonates with acetylacetone fluoro derivatives - trifluoroacetylacetone (TFA) and hexafluoroacetylacetone (HFA) is carried out according to the conditions of examples 1 and 2. The yields of the corresponding products are not lower than: Rh (TFA) ₃ -90%, Rh (HFA) ₃ -88%, Ir (TFA) ₃ -86%, Ir (HFA) ₃ -82%.

Claims (3)

1. The method of obtaining beta-diketonates of iridium (III) and rhodium (III) of the General form
(R ¹ -CO-CH-CO-R ^{1 1} ) ₃ M,
where M Rh, Jr;
R ¹ , R ^{1 1} CH ₃ , CF ₃ ,
comprising dissolving the starting complex compound of platinum metal in hydrofluoric acid and heating the reaction mixture, characterized in that the starting compounds are iridium (III) or rhodium (III) hexafluorocomplex, which are hydrolyzed in an acidic medium, alkalized, an excess of betadiketone is introduced, followed by alkalization of the reaction mixtures and the selection of the target product.  2. The method according to claim 1, characterized in that the hexafluorocomplex of iridium (III) or rhodium (III) is obtained by fusion of the hexanitrite complex of the corresponding metal with a mixture of potassium bifluoride with hydrofluoric acid in a mass ratio of 1 0.5 to 1.0.  3. The method according to claims 1 and 2, characterized in that, depending on the metal and type of ligand, alkalization is carried out to pH 3 7.